S. Schulze (PUC, MAS), T. Kruehler (ESO Chile), D. Xu, J. P. U. Fynbo (DARK/NBI), N. R. Tanvir (U Leicester), P. Jakobsson (U. Iceland), S. Galleti (INAF-OAB) report on behalf a larger collaboration:
We observed the field of GRB 141004A (D'Elia et al. GCN 16878, Mereghetti et al. GCN 16879) with the 2.5-m Nordic Optical Telescope (NOT) equipped with ALFOSC. Observations started at 01:52:22 UT on October 5 (i.e., 2.52 hr after the burst). We obtained a 200-s image in the R band.
The afterglow reported in D'Elia et al. (GCN 16878) and Gorosabel et al. (GCN 16880) is clearly detected. It has an R-band magnitude of 20.6 mag (not corrected for foreground extinction), calibrated against several USNO stars. The mid-exposure time is 2.55 hr after the trigger. Compared to D�Elia et al (GCN 16881), the brightness of the afterglow did not decrease.
In addition, we obtained an optical spectrum, with a total exposure of 3 x 1800 s, using Grism #4 and covering the wavelength range 3750 - 9000 AA at a resolution of 17 AA. Observations started at 02:17:23 UT (i.e. 2.9 hr after the burst). The continuum is detected down to ~3750 AA. No absorption line is visible in the spectrum. A tentative emission line is detected at 5866.4 AA, which could be [OIII]-5007 at z=0.17 or [OII] at z=0.57. A higher S/N spectrum is needed to elucidate the nature of this feature.